1. Field of the Invention
The present invention relates to a picture data processing device and, more particularly, to a device for coding picture data by compression using bidimensional orthogonal transform, and a method thereof.
2. Description of the Related Art
It is a common practice with a digital electronic still camera or similar still picture processing system to convert an analog picture signal to digital picture data, compress the picture data to an amount smaller than a desired amount, and record the compressed picture data in a semiconductor memory, magnetic disk, optical disk or similar picture data recording medium. A bidimensional orthogonal transform system, for example, divides one frame of picture data into a predetermined number of blocks each having a predetermined size and converts each block of picture data to data of frequency domain, i.e., transform coefficients. Among the transform coefficients, the coefficients smaller than a predetermined threshold are discarded while the other coefficients are divided by a predetermined quantizing step, i.e., normalizing coefficient to be thereby normalized. The normalized coefficients are compressed by run length coding and Huffman coding and then written to a picture data storing medium. This kind of procedure is successful in using the capacity of the storing medium efficiently. For details of the bidimensional orthogonal transform and other compression coding schemes, a reference may be made to, for example, Japanese Patent Application No. 309870/1988 having the same applicant as the present application.
A prerequisite for the still image processing system is that a desired number of picture data be stored in a memory card or similar picture data storing medium having a limited capacity. To meet this requisite, Japanese Patent Application No. 27499/1989 having the same applicant as the present invention, for example, allocates a particular bit distribution value to each block of one frame of picture data in relation to an activity, sequentially compresses low frequency components to high frequency components in this order, and stops the compression as soon as the amount of codes reaches the bit distribution value. While this approach successfully maintains the amount of compressed codes constant, it cannot control the amount of codes in a delicate way. Specifically, it is likely that some of the storage locations available for recording, for example, one frame of picture data are left idle, resulting in limited efficiency. Moreover, once the amount of codes distributed on the basis of an activity is deviated from a predicted amount, the number of pictures which can be stored in a recording medium is reduced. In addition, such control over the amount of codes is not practicable without resorting to a large scale and complicated circuit arrangement.